Contact element



March 25,1947. J.-M, 500E 2,417,967

CONTACT ELEMENT Filed Feb. 23, 1944 'INDIIIM J-E-J'IL FER fig 3 us: new:

Iii-mama I J7-suvznf V z'ywvnlun 21-11mm! 27-sarzn 28 2'6 Z4-nvo1un 25 Jena 1% .5006 1114 50 BY 22 I 3 a I ZZ-COPPER flfrOR/VE) Patented Mar. 25, 1947 CONTACT ELEMENT James M. Rona-Indianapolis, Ind,

assignor to P. R. Mallory & 00., Inc., Indianapolis, Ind., a

corporation of Delaware Application February 23, 1944, Serial No. 523,547

4 Claims. 1

The'present' invention relates to electric contact members such as wiping or sliding contacts, switch contacts and the like.

An object of the invention is to improve the operating characteristics of electric contact elements.

Other objects of the invention will become apparent from the following description taken in conjunction with the accompanying drawing in which:

Figure 1 shows co-operating switch contacts embodying features of the invention;

,Figure 2 is a cross-section alOng line 2-2 of Figure 1;

Figures 3a and 3b are sections of wire cmbodying features of the invention;

Figure 4 is a fragmentary section of a slip ring contact embodying features of the invention, together with a co-operating sliding brush contact; and

Figure 5 shows an electric make-and-break contact embodying a modification of the invention.

Referring to Figures 1 and 2, there is shown a portion of a rotary switch of the type illustrated in more detail in U. S. Patent No. 2,177,483, issued October 24, 1939, to Harry C. Hall, which is adapted to be used for changing circuit connections in radio sets and other electricalapparatus. The switch comprises a rotor contact shoe l0, and stator contact members it and I2, respectively, said shoe l0 being secured to insulating rotor disc 13 which is mounted to rotate about the center 0. Stator contacts I! and I2 comprise folded metal strips which are secured to insulating stator plate M by metal'straps I5. The ends of each strip where they are brought together by folding are formed into a co-operating pair of contact jaws l6 which engage opposite faces of rotor shoe l0 near its outer arcuate edge. It is obvious that when the rotor is turned, rotor shoe ID will rotate and slide within jaws IE to electrically connect and disconnect the stator contacts depending upon the position to which the rotor is turned.

For radio and electronic circuits it is of considerable importance that the contacts have uniformly low electrical resistance at all times and under all conditions of operation. In some cases it is necessary that the switches operate satisfactorily at temperatures as low as 70 below zero Fahrenheit.

For low resistance it is desirable that good contact pressure be applied by the jaws l6 against the rotor shoe It]. On the other hand higher pressures increase abrasion and wear and shorten the life of the switch. They also increase the drag so that where several switch sections are ganged together under control of a single knob and indexing mechanism the torque required to operate the switch is undesirably high and also interferes with positive action of the index mechamsm. 7

Silver and silver alloy contacts have been used for some time for such switches and have been fairly satisfactory where requirements are not too severe. However, it has heretofore been im possible, in spite of extensive experimental work, to satisfactorily meet all the requirements for certain applications, such as military and air craft uses, where good operation must often be maintained even at very low temperatures.

Application of an organic lubricant to silver contacts helps to maintain low contact resistance and increase the life of the switch. However, it does not greatly reduce the drag, hence such switches have an objectionably high torque.

According to the present invention a layer of indium metal is applied to the surface of one or both of the co-operating contacts. it is generally adequate to apply the indium coating to one of the contacts of the combination, preferably the one having the larger contact area. The indium is preferably applied to a contact of pure silver, although silver alloys such as coin silver, sterling silver, silver copper-nickel-cadmium alloys and the like may sometimes be used. An organic lubricant is preferably applied over the indium.

Referring again to the drawing, particularly Figure 2, the contact shoe It] may be formed of a brass base "I plated with a silver plate 8 and then with a thin layer 9 of indium. Only a very thin layer of indium is required, which may be in the order of 1m 100 microinches in thickness. The preferred range is 5 to 20 microinches. Preferably the silver layer 8 may be left with a matte surface, and the indium is applied over this matte tumbling with abrasive particles.

Sodium dichromate ounces 3 Sulfuric acid pints Water to make 1 gallon.

After this dip the parts are rinsed in cold water and treated for a few seconds (until the red coloration is removed) in a 10% solution of sulphuric acid. After this the parts are thoroughly rinsed and then dipped in a 5% solution of sodium cyanide followed by an additional rinse and the parts are'then ready to receive the indium deposit.

It is believed that the wiping action resulting from contact operation wipes the indium into the minute recesses in the silver surface and exposes minute silver peaks. The contact members II and 12 are preferably made of silver plated brass or of solid silver alloys of sufiicient stiffness. Hence a direct silver-to-silver contact can be obtained.

The life of the switch can be further extended by coating the contacts with a thin film of organic lubricant. For uses where extremely low temperatures are encountered Mobilgrease Zero" has been found suitable. Other low temperature lubricants having an aluminum soap base can also be used. For other uses lanum is suitable.

The invention is also applicable to slide wire variable inductance devices of the type described in Paul Ware Patent 2,163,645 granted June 27, 1939. For example, the inductance coil upon which the moving contact slides, as shown in Figure 3a, may be silver wire I! plated with indium 18 or as shown in Figure 3b, copper wire [9 plated with silver 20, then with indium 2|. An organic lubricant may also be used over the indium surface.

Figure 4 is a section showing the invention applied to a slip ring which may be used as a sliding contact for various electrical components, such as the end contact rings of slide wire variable inductance devices. The ring 22 may be of good conductivity metal.such as copper, plated on the contacting surface with a layer 23 of silver, which in turn is plated with a thin indium layer 24. This co-operates with a sliding contact 25 which may have a silver or other metal facing 26.

Figure 5 shows an electric contact 21 of silver attached to an arm 28. The contact surface is given a layer of indium 29. This may be used as a Wiping contact or make-and-break contact.

The contacts can be made by conventional electroplating processes for applying silver and indium. Methods of indium plating are described in Gray Patent 1,935,630 and Murray et al. Patent 1,965,251. Some diffusion of the indium into the silver base may take place at higher contact operating temperatures, or may be effected by a short moderate temperature heat treatment to improve the bond. However, an intentional diffusing operation is not generally necessary. Complete diffusion of the indium into the silver lowers the lubricating value and hence it is preferable that an indium layer be preserved on the surface.

While specific embodiments of the invention have been described, it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A sliding contact having a contacting portion of substantially pure silver with a thin coating of indium thereon, said coating of indium having a thickness of about 5 to about 20 microinches and being substantially free from silver on at least the exposed surface thereof.

2. A wiping contact member comprising a body portion composed of a relatively stifl resilient metal, a thin layer of silver electrodeposited on said body portion, and a coating of indium having a thickness between about 5 and about 20 microinches electrodeposited on said silver layer and bonded thereto whereby the wiping action resulting from contact operation will wipe the indium into the minute recesses in the silver surface.

3. A wiping contact member comprising a body portion composed of resilient metal, a thin layer of "silver on said body portion, and a coating of indium electrodeposited on said silver layer and bonded thereto, said coating of indium having. an extremely minute thickness in the order of about 5 to about 20 microinches whereby as a result of such minute thickness of the indium coating the detrimental eflect of the high specific resistivity of indium upon the contact resistance is substantially eliminated while the beneficial lubricating effect of indium upon the smooth contact operation is substantially fully retained.

4. A wiping contact member comprising a body portion of base metal, a thin layer of silver on said body portion having a matte surface as a result of the presence of microscopic recesses therein, and a coating of indium on said silver layer having a thickness in the order of 1 to microinches sufilcient forfillingout said recesses.

JAMES M. BOOE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,847,941 Gray Mar. 1, 1932 1,934,730 Murray Nov. 14, 1933 2,303,497 Reeve Dec. 1, 1942 OTHER REFERENCES Metals and Alloys," March 1942, page 498. (Copy in Div. 3.) 

